The present invention relates to a two-frequency printed antenna that is used as a base station antenna in a mobile communication system, and is used in common for two frequency bands which are separated apart from each other, and to a multi-frequency printed antenna used in common for a plurality of frequency bands which are separated apart from each other, and to a two-frequency or multi-frequency array antenna composed of the two- or multi-frequency printed antennas.
Antennas such as base station antennas for implementing a mobile communication system are usually designed for respective frequencies to meet their specifications, and are installed individually on their sites. The base station antennas are mounted on rooftops, steel towers and the like to enable communications with mobile stations. Recently, it has been becoming increasingly difficult to secure the sites of base stations because of too many base stations, congestion of a plurality of communication systems, increasing scale of base stations, etc. Furthermore, since the steel towers for installing base station antennas are expensive, the number of base stations has to be reduced from the viewpoint of cost saving along with preventing spoiling the beauty.
The base station antennas for mobile communications employ diversity reception to improve communication quality. Although the space diversity is used most frequently as a diversity branch configuration, it requires at least two antennas separated apart by a predetermined distance, thereby increasing the antenna installation space. As for the diversity branch to reduce the installation space, the polarization diversity is effective that utilizes multiple propagation characteristics between different polarizations. This method becomes feasible by using an antenna for transmitting and receiving the vertically polarized waves in conjunction with an antenna for transmitting and receiving the horizontally polarized waves. In addition, utilizing both the vertically and horizontally polarized waves by a radar antenna can realize the polarimetry for identifying an object from a difference between radar cross-sectional areas caused by the polarization.
Thus, to make effective use of space, it is necessary for a single antenna to utilize a plurality of different frequencies, and in addition, the combined use of the polarized waves will further improve its function. FIG. 1 is a plan view showing a conventional two-frequency printed antenna disclosed in Japanese patent application laid-open No. 8-37419/1996. FIG. 2 is a schematic view showing a configuration of a conventional antenna formed as a corner reflector antenna comprising the two-frequency array antenna. In this figures, the reference numeral 101 designates a dielectric board; 102a designates a dipole element printed on the first surface of the dielectric board 101; 102b designates a dipole element printed on the second surface of the dielectric board 101; 103a designates a feeder printed on the first surface of the dielectric board 101; 103b designates a feeder printed on the second surface of the dielectric board 101; 104 designates a passive parasitic element; 105 designates reflectors joined to each other; 106 designates a corner reflector composed of two reflectors 105 joined; and 107 designates subreflectors joined to both ends of the corner reflector 106. The right and left dipole elements 102a and 102b constitute a dipole antenna 102 operating at a particular frequency f1; and the two feeders 103a and 103b constitute a twin-lead type feeder 103. The parasitic element 104 has a length resonating at a frequency f2 higher than the frequency f1. The antenna as shown in FIG. 2 is a side view of a device configured by adding the corner reflector to the dipole antenna as shown in FIG. 1. In FIG. 2, the dipole antenna 102 and the twin-lead type feeder 103 are shown schematically.
Next, the operation of the conventional antenna will be described.
The dipole antenna has a rather wideband characteristic with a bandwidth of 10% or more. To achieve such a wide bandwidth, however, it is necessary for the height from the reflectors to the dipole antenna to be set at about a quarter of the wavelength of the radio wave or more. Besides, since the dipole antenna forms its beam by utilizing the reflection from the reflectors, when the height to the dipole antenna is greater than a quarter of the wavelength, it has a radiation pattern whose gain is dropped at the front side. Therefore, it is preferable that the height from the reflectors to the dipole antenna be set at about a quarter of the wavelength of the target radio wave.
In the conventional antenna, the dipole antenna 102 fed by the feeder 103 resonates at the frequency f1. When the dipole antenna 102 operates at the frequency f2 higher than the frequency f1, the parasitic element 104 disposed over the dipole antenna 102 resonates at the frequency f2 because of the induction current caused therein by inter-element coupling. Therefore, the dipole antenna 102 and the parasitic element 104 thus arranged can implement two-frequency characteristics. In addition, the beam width can be controlled by utilizing reflected waves from the corner reflector 106 and subreflector 107.
With the foregoing configuration, the conventional antenna can operate at both frequencies f1 and f2. However, the parasitic element 104, which is active at the relatively high frequency f2 and is disposed over the dipole antenna 102 operating at the relatively low frequency f1, presents the following problems: First, it is impossible for the dipole antenna 102 and the parasitic element 104 to be placed at the height of a quarter wavelength of the radio waves of the operating frequency at the same time. Second, because of the effect of the current flowing in the dipole antenna 102 even when the parasitic element 104 is active at the frequency f2, it is difficult to obtain similar beam shapes by controlling the beam width at the frequency f1 and f2. In addition, the corner reflector and subreflectors needed to achieve the beam control present another problem of complicating the structure of the antenna.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide a two-frequency antenna, a multi-frequency antenna, and a two-frequency or multi-frequency array antenna composed of the foregoing antennas, which can obtain similar beam shapes at individual operating frequencies when the single antenna is used in common for a plurality of operating frequencies.
Another object of the present invention is to provide a two-frequency antenna, a multi-frequency antenna, and a two-frequency or multi-frequency array antenna composed of the foregoing antennas, each of which has a simple structure and can be used in common for a plurality of operating frequencies.
According to a first aspect of the present invention, there is provided a two-frequency antenna comprising: a feeder, an inner radiation element connected to the feeder and an outer radiation element, all of which are printed on a first surface of a dielectric board; an inductor formed in a gap between the inner radiation element and the outer radiation element printed on the first surface of the dielectric board to connect the two radiation elements; a feeder, an inner radiation element connected to the feeder and an outer radiation element, all of which are printed on a second surface of a dielectric board; and an inductor formed in a gap between the inner radiation element and the outer radiation element printed on the second surface of the dielectric board to connect the two radiation elements.
Thus, the two-frequency antenna can operate at the frequency f1 at which the sum length of the inner radiation element, the inductor and the outer radiation element becomes about a quarter of the wavelength. As for the frequency f2 at which the length of the inner radiation element becomes about a quarter of the wavelength, the two-frequency antenna can also operate at the frequency f2 higher than the frequency f1 by matching the resonant frequency of the parallel circuit, which consists of a capacitor based on the capacitive gap and the inductor, to the frequency f2. Therefore, the single antenna can achieve the function of two linear antennas, each having a length of half the wavelength of the radio wave with one of the frequencies f1 and f2. This offers an advantage of being able to implement the two-frequency antenna with the radiation directivity with the same beam shape for the two different frequencies. In addition, since the resonant length that determines the resonant frequency of the linear antenna includes the length of the inductor, the linear antenna has an advantage over an ordinary linear antenna with the same resonant frequency that its size can be reduced.
According to a second aspect of the present invention, there is provided a multi-frequency antenna comprising: a feeder, an inner radiation element connected to the feeder and a plurality of other radiation elements separated apart from each other, all of which are printed on a first surface of a dielectric board; a plurality of inductors, each of which is formed in a gap between adjacent radiation elements printed on the first surface of the dielectric board to connect the two adjacent radiation elements; a feeder, an inner radiation element connected to the feeder and a plurality of other radiation elements separated apart from each other, all of which are printed on a second surface of a dielectric board; and a plurality of inductors, each of which is formed in a gap between adjacent radiation elements printed on the second surface of the dielectric board to connect the two adjacent radiation elements.
This makes it possible for a linear antenna to operate at a resonant frequency f, wherein the linear antenna consists of the antenna elements each of which includes one or more radiation elements and zero or more inductors inside any pair of the corresponding gaps formed on the first and second surfaces, and f is the resonant frequency of the linear antenna, by matching the resonant frequency of the parallel circuit, which consists of the inductors connecting the gaps and capacitors equivalent to the capacitive gaps, to the frequency f. Therefore, the single antenna can operate at three or more operation frequencies by making a set as described above. This offers an advantage of being able to implement the multi-frequency antenna with the radiation directivity with the same beam shape for the three or more different frequencies. In addition, since the resonant length that determines the resonant frequency of the linear antenna includes the length of the inductor, the linear antenna has an advantage over an ordinary linear antenna with the same resonant frequency that its size can be reduced.
Here, the inductor, which is formed in the gap between the inner radiation element and the outer radiation element printed on the first surface of the dielectric board to connect the two radiation elements, may employ a strip line printed on the first surface of the dielectric board as the inductor; and the inductor, which is formed in the gap between the inner radiation element and the outer radiation element printed on the second surface of the dielectric board to connect the two radiation elements, may employ a strip line printed on the second surface of the dielectric board as the inductor.
Since the linear antenna can be formed integrally on the dielectric board by the etching process, it has an advantage of being able to be fabricated at high accuracy with ease.
The inductors, which are formed in the gap between the adjacent radiation elements printed on the first surface of the dielectric board to connect the two adjacent radiation elements, may employ a plurality of strip lines printed on the first surface of the dielectric board as the inductors; and the inductors, which are formed in the gap between the adjacent radiation elements printed on the second surface of the dielectric board to connect the two adjacent radiation elements, may employ a plurality of strip lines printed on the second surface of the dielectric board as the inductors.
Since the linear antenna can be formed integrally on the dielectric board by the etching process, it has an advantage of being able to be fabricated at high accuracy with ease.
The two-frequency antenna may further comprise a notch formed at an intersection of the inner radiation element and the feeder formed on the first surface of the dielectric board; and a notch formed at an intersection of the inner radiation element and the feeder formed on the second surface of the dielectric board.
This makes it possible to change the passage of the current flowing in the inner radiation elements, and hence offers an advantage of being able to shift the operating frequency of the linear antenna to a lower range with little varying the other operating frequency, when the inner radiation elements are considered to be the antenna elements of the linear antenna.
The multi-frequency antenna may further comprise a notch formed at an intersection of the inner radiation element and the feeder formed on the first surface of the dielectric board; and a notch formed at an intersection of the inner radiation element and the feeder formed on the second surface of the dielectric board.
This makes it possible to change the passage of the current flowing in the inner radiation elements, and hence offers an advantage of being able to shift the operating frequency of the linear antenna to a lower range with little varying the other operating frequencies, when the inner radiation elements are considered to be the antenna elements of the linear antenna.
The two-frequency antenna may consist of a xcex9-shaped linear antenna or a V-shaped linear antenna, wherein the xcex9-shaped linear antenna may comprise an antenna element consisting of the inner radiation element, the inductor and the outer radiation element, which are formed on the first surface of the dielectric board, and an antenna element consisting of the inner radiation element, the inductor and the outer radiation element, which are formed on the second surface of the dielectric board, the two antenna elements forming an angle less than 180 degrees at a side of the feeder; and wherein the V-shaped linear antenna may comprise the antenna element formed on the first surface of the dielectric board, and the antenna element formed on the second surface of the dielectric board, the two antenna elements forming an angle greater than 180 degrees at the side of the feeder.
This offers an advantage of being able to adjust the beam width of the linear antenna in accordance with its application purpose when operating it at the relatively low operating frequency f1 and the relatively high operating frequency f2.
The multi-frequency antenna may consist of a xcex9-shaped linear antenna or a V-shaped linear antenna, wherein the xcex9-shaped linear antenna may comprise an antenna element consisting of the plurality of radiation elements and the plurality of inductors, which are formed on the first surface of the dielectric board, and an antenna element consisting of the plurality of radiation elements and the plurality of inductors, which are formed on the second surface of the dielectric board, the two antenna elements forming an angle less than 180 degrees at a side of the feeder; and wherein the V-shaped linear antenna may comprise the antenna element formed on the first surface of the dielectric board, and the antenna element formed on the second surface of the dielectric board, the two antenna elements forming an angle greater than 180 degrees at the side of the feeder.
This offers an advantage of being able to adjust the beam width of the linear antenna in accordance with its application purpose when operating it at the relatively low operating frequency f1 and the relatively high operating frequency f2.
The two-frequency antenna may further comprise a ground conductor with a flat surface or curved surface, and a frequency selecting plate with a flat surface or curved surface, wherein the linear antenna may be installed at a position separated apart from the ground conductor by about a quarter of a wavelength of a radio wave with a relatively low operating frequency f1, and the frequency selecting plate may be installed at a position separated apart from the linear antenna by a quarter of a wavelength of a radio wave with a relatively high operating frequency f2, on a side closer to the ground conductor and in substantially parallel with the ground conductor.
This offers an advantage of being able to maximize the gain at the front of the antenna at the two operating frequencies because the height of the linear antenna becomes about a quarter of the wavelength of the radio wave for the individual operating frequencies f1 and f2.
According to a third aspect of the present invention, there is provided a two-frequency array antenna comprising a plurality of two-frequency antennas as defined above, which are arranged in a same single direction or in orthogonal two directions.
As for the two-frequency antenna, this offers an advantage of being able to implement a single polarization two-frequency array antenna or an orthogonal two-polarization two-frequency array antenna, which has the foregoing advantages such as achieving the radiation directivity with the same beam shape for two different frequencies.
According to a fourth aspect of the present invention, there is provided a multi-frequency array antenna comprising a plurality of two-frequency antennas as defined above, which are arranged in a same single direction or in orthogonal two directions.
As for the multi-frequency antenna, this offers an advantage of being able to implement a single polarization multi-frequency array antenna or an orthogonal two-polarization multi-frequency array antenna, which has the foregoing advantages such as achieving the radiation directivity with the same beam shape for two different frequencies.